Various means and methods for communicating between a stand-alone device and a host environment without the need for direct wire connections are currently known in the art. An example of such stand-alone devices are integrated-circuit storage devices, usually of the non-volatile type, which may store business transaction information, such as credit card information. An example of a host environment which would operate with such a credit card would be an electronic information processing device which utilizes and updates the information stored by the above-mentioned integrated-circuit storage devices. Another example of such a stand-alone device would be a security pass card which, upon being powered, would output a digital "password" to a corresponding host environment for security clearance. The advantage of these wireless systems over conventional magnetic-strip information bearing cards and the like is that they provide a more reliable communications channel between the host and the stand-alone device (e.g., card) because they are "contactless" systems. However, these wireless systems are more expensive to manufacture and have different reliability problems, as discussed below in grater detail.
Such prior art "wireless" stand-alone devices are generally constructed with a plurality of I.C. chips and magnetic coil components disposed on a substrate carrier (e.g., "credit card") using hybrid circuit construction techniques. This is because relatively complicated communications techniques are used by these wireless devices, which require multiple coils and multiple processing chips. For example, U.S. Pat. No. 4,605,844 to Haggan teaches a hybrid construction of two integrated circuit chips, three magnetic coils, and three separate communication channels for power, incoming data, and outgoing data in a stand-alone device. U.S. Pat. No. 4,720,626 to Nishikawa, et al., teaches a hybrid construction for a stand-alone device including an I.C. chip, a first coil for clock and power, a second coil for outputting data, and a hall-effect device for receiving data. As with Haggan, three separate "channels" are used: power/clock, data in, and data out. U.S. Pat. No. 4,791,285 to Ohki also teaches a hybrid construction including an I.C. chip and four coils (power, data in, data out, mode command).
Examples of wireless stand-alone devices using a single coil are provided by U.S. Pat. Nos. 4,388,524 and 4,473,825, both issued to Walton. In each example, power is periodically coupled to the coil in the stand-alone device from the host system. In each, code sequences can be communicated by the stand-alone device to the host system using the same coil. In the '524 patent, a variable resonant frequency circuit is formed with the coil and a variable capacitor. The resonance frequency is modified according to the code sequence and is detected by the host system. In the '825 patent, signals of different frequencies are selectively applied to the coil according to the code sequence and detected by the host system. The patents do not disclose the capability of communicating data in the reverse direction from the host system to the stand-alone device. In these examples, the time required for transmitting the code sequence is much greater than the time allocated for receiving power. This naturally requires a significant energy storage capacity in the stand-alone device. As stated in the '825 patent, an electrolytic capacitor or storage battery is used to store energy provided by the power pulses, thus indicating a hybrid construction. The need for such a large power storage capacity teaches away from the formation of these systems on an integrated circuit (IC) chip (i.e., a substrate having dimensions on the order of one centimeter per chip side and less), as is done in the present invention. This is because present day IC chips can only provide a limited amount of capacitive power storage capability on the order of a few hundred picofarads, which is several orders of magnitude less than the storage capability of electrolytic capacitors and storage batteries.
Additionally, the physical size of the coils taught in the '524 and '825 patents are relatively large and are configured as antennas disposed on credit-card size cards (i.e., 5.5 cm by 8.5 cm). In view of the need of these circuits to obtain a large mount of power in a relatively short time duration and based on a realistic estimate of the power coupling to the coil antennas used in these patents, it would appear to one of ordinary skill in the art that the size of these coils is on the order of a credit card (5.5 cm by 8.5 cm). The need for large coil antennas also teaches away from the formation of these systems on an integrated circuit (IC) chip, as is done in the present invention. This is because the area that the largest present day IC chip can provide for such a coil antenna is roughly 1/45.sup.th of the area provided by a credit card. In contrast to the circuitry of these patents, the present invention provides a communication means which is more power efficient than those used by these patents and enables the present invention to be integrated on an IC chip.
On a related point, these examples have relatively complex circuitry in the host environment due to the use of frequency modulation transmission. This tends to raise the cost of manufacturing the circuitry for the host.
The cost of such hybrid construction for these wireless communication systems is presently too high for the credit-card and identification markets to bear. Additionally, such hybrid systems may be rendered inoperative by the user flexing the carrier substrate ("credit card"), thus breaking the hybrid connectors between the components. Given the high volume of credit cards, security pass cards, and the like, and given the better communication interface of such wireless systems, there is a great need to decrease the cost of such wireless credit card carriers and to correspondingly increase their durability. The present invention is directed towards these goals.
An example of an integrated circuit chip communication system which is directed towards addressing the disadvantages of hybrid construction is described in the monographs by Adam C. Malamy, "A Magnetic Power and Communications Interface for Pinless Integrated Circuits", Massachusetts Institute of Technology, September 1987, and by Charles W. Selvidge, "A Magnetic Communication Scheme for Integrated Circuits", Massachusetts Institute of Technology, June 1987. These monographs describe a system in which two coils are integrated on a single IC chip, a first coil for receiving a combined power and clock signal from a first electromagnetic coupling medium and a second coil for receiving and transmitting data to and from a second separate electromagnetic coupling medium. To communicate information from the host to the IC chip, the host couples an amplitude modulation signal to the second coil. To communicate information from the IC chip to the host, the IC chip selectively shorts its second coil to magnetically load a corresponding coil in the host. This changes the inductance of the host's coil, which may be detected. An on-chip power supply is generated from the first coil. An on-chip power supply of .about.3 VDC at 0.9 mW was achieved (0.3 mA). Unfortunately, this level of power is not sufficient for most applications of IC wireless communication applications. In this regard, the monographs do not appear to suggest deriving power from the amplitude-modulated signal coupled to the second coil. From their results, however, it would not appear that deriving power from the second coil would be fruitful due to the ON/OFF amplitude modulation of the signal and the low level of power derived from the first coil.
In contrast to the present invention, other prior art wireless communications systems use a finely tuned resonant L-C circuit in the stand-alone device to increase the amount of power that can be coupled to the stand-alone device. In these systems, energy is received by the tuned L-C circuit much more efficiently at or near the L-C resonance frequency. The frequency of the power signal from the host or master station must be tightly controlled to be within a narrow range of frequencies about the resonant frequency. To communicate data information along with the resonant power signal, these prior art systems generally employ frequency modulation of the master station's power signal about this narrow frequency range. These systems do not employ selective gating of electromagnetic energy pulses in the master station's power signal as such gating would cause the frequency of the master's power signal to significantly deviate from its narrow frequency range. This, in turn, would significantly interrupt both the communication of data to the stand-alone device and the coupling of power to the stand-alone device. In this sense, as will become apparent in view of the present invention, these prior an systems teach against the communication system according to the present invention.